In Washington, a sunny day to mark the arrival of autumn

September 22, 2013

It may sound like the model number of a new airplane, but the time autumn arrived in Washington on Sunday afternoon was 4:44.

That was the time of the autumn equinox, the start of astronomical autumn, a moment foreordained by the movement of Earth around the sun, and not necessarily by a sudden avalanche of falling leaves, or the abrupt onset of a chill in the air.

It seemed to be a fine example of a September day in Washington, at least in terms of weather. Bright sunshine, a high temperature in the 70s, with white clouds sailing across a blue sky. And notably dry. That was in sharp contrast to Saturday, when rain fell for hours and hours, making the day Washington’s wettest in weeks and weeks.

At Reagan National Airport, Saturday’s showers, which started mid-afternoon and continued until late at night, brought o.87 inches of rain. That was almost five times as much as any previous day this month, and almost twice as much as on any day this August.

It outlasted the determination of the Washington Nationals baseball team to wait it out. Saturday night’s game was finally called off just before 11 and rescheduled for Sunday night.

It was the most rain on record at Reagan since July 12.

At Dulles International Airport, Saturday’s rain was even heavier than at National. The figure was 1.30 inches. More than half of it fell in a single hour.

But that was the last of the summer rain.

On Sunday, the story was of the motion of Earth around the sun, and the tilt of Earth’s axis. The tilt remains the same throughout the year. But on two days a year there comes a moment that could be said to show axial neutrality.

In summer, in the northern hemisphere, the axis tilts toward the sun, and in winter, away from it. But for a moment on the day of the autumn equinox, just as on the day of the spring equinox, the tilt is neither toward nor away.

At the equinoxes, autumn and spring, daytime and nighttime are each about 12 hours long, as suggested by the “equi” in the word equinox.

With the autumn equinox having passed, Washington and the rest of the northern half of the planet began to tilt away from the sun, heading into the autumn and toward the winter.

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